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Swift/XRT radiation damage and results from the lab using a proton-damaged CCD

Claudio Pagani (Swift/XRT team)
XMM Calibration Meeting, Leicester, 7/3/2012


XRT CCD


e2v CCD-22 detector (developed for EPIC MOS camera on XMM) Operated in Photon Counting (PC) and Windowed Timing (WT) mode 4 55Fe corner sources continuously illuminate CCD corners, used to monitor CCD performance Spectral resolution at launch: FWHM = 135 eV at Mn K- (5.9 keV) Swift in Low-Earth orbit, exposed to high flux of protons (South Atlantic Anomaly)










XRT Radiation Damage


XRT Trap mapping
Trap mapping: map pixels affected by radiation damage, measure the charge losses of individual pixels


Cas A, Tycho SNR offset pointings to cover (partially) CCD area Silicon line (1.85 keV) as reference energy, fit line to localize traps and measure trap depths




Trap localization

BOTTOM

TOP


Recovered energy resolution


Trap depths = f(T)


Temperature
XRT CCD relies on passive cooling after TEC failure, operating between -75 and -50C Dark current, shorter emission line fills traps at higher T Average CTI is measured to be temperature-dependent Temperature dependence seen in calibration corner sources analysis


Trap depths = f(E,Flux)


Energy dependence
Larger charge cloud will interact with more traps Averaged energy dependence measured using emission lines in Tycho, E0102, but lines too weak to measure it for each damaged pixel



Source flux dependence
"Sacrificial" charge effect No bright source with emission lines to test the effect on-board the XRT


Laboratory experiment


Experimental set-up


SRC camera test facility
Space Research Centre camera test facility

Thanks to David Vernon, SRC


Proton-damaged CCD
Copy of XRT e2V CCD-22 irradiated with 10 MeV proton beam at Harwell tandem accelerator facility

Dose of 5x108 10 MeV protons Dose of 2.5x108 10 MeV protons


Laboratory datasets
High statistics datasets at selected energies and CCD operating temperatures:


ENERGY: Oxygen (0.5 keV), Copper (0.9 keV), Aluminium (1.2 keV), Silicon (1.8 keV), Titanium (4.5 keV) and Iron (6.4 keV) TEMPERATURE: Camera cooled at set of temperatures comparable to Swift/XRT operational range (Ti and Si): CCD_T = [-100, -75, -70, -65, -60, -55, -50C]





CCD uniformly illuminated, 10k frames at each setting, flux of ~ 600 single pixel X-rays/frame.


Laboratory results - CTI
Columns with no large traps Double radiation dose: Shifted in energy Higher noise Higher CTI


Laboratory results - Temperature
-50C

-65C

-75C

-100C


Laboratory results - Temperature

T=-50C T=-60C T=-70C T=-100C


Laboratory results - Temperature

T=-50C T=-60C T=-70C T=-100C -100C

Y

Ytrap

Trap Y =

trapped

Y -

emitted

Y


Y
trap

Y

[ E

trapped

y - E

emitted

y

trapped

y -

emitted

y ]

Eemitted is a function of the emission time constant, depends on CCD temperature


Laboratory results - Temperature

A) Break

B) Turnover

C) Flatter


Lab results ­ Sacrificial charge


Dataset with 3x flux ­ X-rays between trap position and measured X-ray event could fill the trap

Ytrap

E(Si)-Etrap

E(Si)

Y


Lab results ­ Sacrificial charge
3x flux

1x flux

At -75C no "step" in energy profile is seen, but gradual energy "recovery", temission(T=-75)

Laboratory results - Energy
Increasing depth vs energy Saturation effect

Different behaviour of trap depths on energy is an indication of different trap properties (i.e., size of defects in pixels).


Work and analysis in progress
Laboratory


Dataset at high flux at T=-100C to investigate "sacrificial charge" effect Complete analysis, and introduce classification of traps based on their observed properties Possible XRT damage by secondary neutrons Model secondary emission, if significant possibility of irradiation of CCD with neutron beam



Secondary neutrons


Details of XRT trap mapping analysis in Pagani et al. 2011, A&A


Backup slides


Backup Trap properties
Readout time


XRT Windowed Timing mode observations: higher timing resolution (1.8 ms) at the expense of limited spatial information Use of trap info from Photon Counting mode observations does not provide the desired energy correction; also, a single scaling factor for the trap depths did not work Differences between modes show influence of readout times on XRT CCD traps and hints at different trap species






CTI = f(T, E, flux)


Choong-Ki Kim CTI equation
Readout time

Source flux

t PT CTI = N T V V exp - 3 e
Energy




[

t PT NZt 1- exp - - 3 e 3
CCD Temperature



PT e

]

Emission time constant
exp [ E T / kT ] e= n v th N C


Radiation Damage


Swift in Low-Earth orbit, exposed to high flux of protons (South Atlantic Anomaly) Effects of displacement damage seen in the XRT CCD:




hot pixels increased dark current charge trapping sites



Initial strategy to deal with with decreased resolution: broadened RMFs for different epochs Trap energy corrections in gain files (Sept 2007-now)